It is known in the art that encoding data in a biphase code, such as the Manchester format, obviates DC biases which occur with encoding formats not contemplating a symmetrical waveform. Manchester coding represents a logic zero by a transition within a baud period from the positive to the negative supply rail and represents a logic one vice versa. In order to maintain the appropriate transition within the baud period, an inherent frequency change occurs in the waveform when adjacent data symbols represent opposite logic values. So long as adjacent data symbols represent all logical ones or zeros, the transmitted waveform has a frequency (f), producing the so-called skinny bits. When adjacent data symbols represent different logic values, the waveform changes to a frequency of (f/2) producing the so-called fat bits.
Lossy transmission lines have an impedance which generally increases with frequency causing the skinny bits to be more severely distorted in comparison with the fat bits. The disparate treatment between skinny and fat bits produces intersymbol interference on the transmission line resulting in data jitter.
It is known in the art that distortion induced by lossy transmission lines can be equalized by predistorting the data, i.e. modifying the magnitude and energy content, before transmission. Predistortion, alternatively called transmit equalization, engenders generating a waveform having discrete levels in between the logic rails. The waveform is driven to an intermediate level between the logic rails at a time in between data bauds, in anticipation of a subsequent logic level change. The relative power content between fat and skinny bits is equalized by truncating the amplitude of fat bits to an intermediate level located between the logic rails.
The Institute of Electrical and Electronic Engineers (IEEE) in proposed supplement P802.3I/D11, incorporated herein by reference, sets forth standards for a local area network (LAN) having a twisted pair cable as its transmission line. The twisted pair network is commonly referred to as a 10Base-T network. The standards set forth include a stringent budget for allowable data jitter induced by the twisted pair cable, necessitating a line driver employing some form of transmit equalization. In the prior art, two approaches for a line driver employing transmit equalization have been contemplated, each have their own inherent disadvantages.
A first approach embodies a line driver configured in a current driving mode having a differential output coupled through a transmit filter to a primary winding of a 1:1 ratio transformer. The primary winding of the transformer includes a center tap coupled to the positive supply rail, which typically is five volts. In this configuration, the transformer acts as both a load for the current driver and an isolating coupler to the transmission line. An example of this type of driver is the ML4653 device produced by Micro Linear Corporation. A serious limitation with this approach is that the current driver must be realized in bipolar technology in order for it to be reliable. Another disadvantage is that the transformer must include a center tap on the primary winding.
A second approach in the prior art which may enjoy the benefits provided for by CMOS technology, is depicted in simple block diagram form in FIG. 1. The line driver is a voltage driving device for driving large capacitive loads comprising noninverting buffers 10 and 12 and inverting buffers 14 and 16. Buffers 10-16 are selectively energized by encoding logic circuitry 18 in response to data received from the LAN attachment unit interface (AUI) 20. Buffers 10 and 16 are externally coupled through a resistor having a low ohmic value (R) and through a transmit filter 22 to a primary winding of a transformer 24. Buffers 12 and 14 are externally wired ORed with buffers 10 and 16 through a resistor having a high ohmic value, typically on the order of ten times R. When energized, buffers 10 and 16 impress a fullstep voltage close to the span of the supply rails across the transmission line. Buffers 12 and 14 when energized, impress a halfstep voltage across the transmission line. Buffers 10 and 16 are selectively energized when encoding logic 18 detects two adjacent data symbols from the AUI 20 of the same value. Likewise, buffers 12 and 14 are energized when encoding logic 18 detects two adjacent data symbols having different values. The external wired ORed approach has the considerable disadvantage in that the transmission function requires dedicating four pins on the integrated circuit package. The larger pin requirement puts the integrated circuit designer in a quandary as to whether to include more pins on the package or to sacrifice some other function to maintain a lower pin count. Examples of such devices include the Am79C98 integrated circuit by Advanced Micro Devices, the T7220 integrated circuit by AT&T and the NCR92C02 integrated circuit by the NCR Corporation.
In accordance with the principals of the present invention, a line driver employing transmit equalization is disclosed which actively encodes fullstep and halfstep information and provides the resulting voltage onto a single pair of output pins. The present invention overcomes limitations in the prior art by providing a two terminal line driver employing predistortion suitable for use in a LAN, such as, but not exclusive to, a 10 Base-T network. The line driver does not require a coupling transformer with a center tap and is realizable in CMOS technology. Further, the present invention requires less die space on an integrated circuit than prior art solutions and the requirement of less pins over prior art solutions allows for additional features to be integrated within the circuit package.